1. Field of the Invention
The present invention concerns processes for modulating a carrier, more particularly an RF carrier, and in which the amplitude and phase errors engendered during this modulation are compensated for.
It also relates to devices that make it possible to perform such modulation with such compensation. It applies mainly in satellite-based bidirectional multimedia communication systems, in particular those using the Ku band or the Ka band. It also relates to LMDS (standing for Local Multipoint Distribution System) type RF transmission systems.
2. Related Art
RF modulation is conventionally performed using a series of changes of frequency based on one or more local oscillators, this being complex and expensive.
It would be beneficial to dispense with these changes of frequency by performing direct vector modulation. This solution would be both simple and particularly inexpensive to effect. It is however limited by the phase and amplitude imbalances engendered by the modulators, particularly at millimeter frequencies when using relatively big frequency bands. These imbalances stem on the one hand from the static spread in hardware, and on the other hand from various drifting, in particular due to temperature.
To compensate for the static spread due to the hardware, one conventionally uses methods consisting in measuring this spread in the factory and in introducing an inverse spread, for example by programming a PROM specific to each modulator at the digital signal processing level. This method has the drawback of being expensive.
To compensate for the other types of defects, as well as possibly for the static spread, it is also possible to use a dynamic calibration system comprising a vector demodulator, as represented in FIG. 1.
In this figure, a digital signal processing system 101, known by the name DSP, directly calculates the modulation signals I and Q. These are then converted into analogue in converters 102 and 103, amplified in gain-control amplifiers 104 and 105 and then filtered in filters 106 and 107.
A local oscillator 108 delivers a carrier frequency which is multiplied by the signal I in a multiplier 109, and after phase-shifting by a variable phase-shifter 110 and a 90-degree phase-shifter 111 by the signal Q in a multiplier 112.
The output signals from the multipliers 109 and 112 are summed in a summator 113 then amplified in an output amplifier 114.
An output coupler 115 makes it possible to tap off a fraction of the output signal from the amplifier 114 so as to apply it to a vector demodulator 116 which makes it possible to detect the errors of phase and of amplitude.
The amplitude error is applied to the gain controls of the amplifiers 104 and 105 and the phase error to the gain control of the variable phase-shifter 110.
To actually determine the errors it is necessary to send a calibrated signal from time to time, this having the drawback of then interrupting the transmission of the useful signal.
In the frequency domain considered, that is to say the millimeter domain, the phase-shifters, the variable one 110 and also the 90-degree one 111, cannot be made with the desired accuracy, that is to say a degree. Furthermore, the vector demodulator 116 is itself marred by quadrature defects, thereby disturbing the correction. Such an architecture is therefore not conceivable in these millimeter frequencies.